As a result of environmental and other concerns, plastic containers, more specifically polyester and even more specifically polyethylene terephthalate (PET) containers are now being used more than ever to package numerous commodities previously supplied in glass containers. Manufacturers and fillers, as well as consumers, have recognized that PET containers are lightweight, inexpensive, recyclable and manufacturable in large quantities.
Blow-molded plastic containers have become commonplace in packaging numerous commodities. PET is a crystallizable polymer, meaning that it is available in an amorphous form or a semi-crystalline form. The ability of a PET container to maintain its material integrity relates to the percentage of the PET container in crystalline form, also known as the “crystallinity” of the PET container. The following equation defines the percentage of crystallinity as a volume fraction:
      %    ⁢                  ⁢    Crystallinity    =            (                        ρ          -                      ρ            a                                                ρ            c                    -                      ρ            a                              )        ×    100  where ρ is the density of the PET material; ρa is the density of pure amorphous PET material (1.333 g/cc); and ρc is the density of pure crystalline material (1.455 g/cc).
Container manufacturers use mechanical processing and thermal processing to increase the PET polymer crystallinity of a container. Mechanical processing involves orienting the amorphous material to achieve strain hardening. This processing commonly involves stretching an injection molded PET preform along a longitudinal axis and expanding the PET preform along a transverse or radial axis to form a PET container. The combination promotes what manufacturers define as biaxial orientation of the molecular structure in the container. Manufacturers of PET containers currently use mechanical processing to produce PET containers having approximately 20% crystallinity in the container's sidewall.
Thermal processing involves heating the material (either amorphous or semi-crystalline) to promote crystal growth. On amorphous material, thermal processing of PET material results in a spherulitic morphology that interferes with the transmission of light. In other words, the resulting crystalline material is opaque, and thus, generally undesirable. Used after mechanical processing, however, thermal processing results in higher crystallinity and excellent clarity for those portions of the container having biaxial molecular orientation. The thermal processing of an oriented PET container, which is known as heat setting, typically includes blow molding a PET preform against a mold heated to a temperature of approximately 250° F.-350° F. (approximately 121° C.-177° C.), and holding the blown container against the heated mold for approximately two (2) to five (5) seconds. Manufacturers of PET juice bottles, which must be hot-filled at approximately 185° F. (85° C.), currently use heat setting to produce PET bottles having an overall crystallinity in the range of approximately 25%-35%.
In many applications, it is desirable to provide a closure or cap for mating with a finish of the bottle or container. Many such bottle and cap combinations are designed with a Tamper Evident (TE) breakaway band on the cap. Such a band is attached to the cap when initially applied to the corresponding bottle finish and upon opening the bottle for the first time, the band is designed to break away from the cap and thus remain on the bottle. Since the band only breaks away one time, the resulting effect proves whether or not a bottle has been tampered with or more specifically if the cap has been removed prior to the actual customer opening the bottle.
In the finish of a container, the TE band profile includes two diameters, which the standardized Packaging Industry labels as an “A-band” and a “B-band”, respectively. A diameter of the A-band (A-diameter) is typically larger than a diameter of the B-band (B-diameter). The larger A-diameter represents a bulge in the container finish profile that is designed as a catch to stop and prevent the TE band of the cap from riding over the top of it in a reverse direction when the cap is removed from the container. The TE band of the cap is designed to ride over the A-diameter band profile of the container in a forward (downward) direction when the cap is initially applied to the container. The prevention of the band moving back up on the bottle when the cap is removed thus creates the necessary engagement interface and force that effectively removes the breakaway band from the cap and hence leaves it on the container finish.
Within the realms of the PET blow molding industry, where it is desirable to convert injection molded PET preforms into blow molded PET containers, it has been shown that any blow moldable detail that is designed and built into any given blow mold, be sized in such a way that ensures duplication of that mold detail onto the moldable surface of the given container. In the blow molding industry, it is generally accepted as being very difficult to achieve a desired blown definition of the actual mold surface when multiple ribs, channels, etc., are bunched close to each other on the actual mold surface. As a result, during the blow molding process, the TE band of the container finish may form properly while the thread run-out, or portion of the thread that is most closely positioned to the TE band, may not always form properly.